It is known that the human .alpha..sub.2 -plasmin inhibitor (to be abbreviated hereinafter as "human .alpha..sub.2 -PI") is a single-chain glycoprotein having a carbohydrate content of 11.7% and a molecular weight of about 67,000 which was first isolated in pure form from human plasma by Aoki and Moroi and acts as a strong inhibitor capable of instantaneously inhibiting the esterase activity of plasmin, a fibrinolytic enzyme [see Moroi & Aoki: The Journal of Biological Chemistry, 251, 5956-5965 (1976)].
On the other hand, human .alpha..sub.2 -PI has three functions. Firstly, it has a site of inhibiting the fibrinolytic activity of plasmin (in the present specification, this site is referred to as the "reactive site") [see B. Wiman & D. Collen: The Journal of Biological Chemistry, 254, 9291-9297 (1979)]. Secondly, it has a site combining with plasmin at the carboxyl group terminal [B. Wiman & D. Collen: European Journal of Biochemistry, 84, 573-578 (1978)]. Thirdly, it has a site combining with fibrin at the amino group terminal [Y. Sakata et al.: Thrombosis Research, 16, 279-282 (1979)].
If it is possible to provide a monoclonal antibody which selectively blocks the reactive site of human .alpha..sub.2 -PI among these three active sites, it will be very interesting in medicine for the treatment of thrombotic diseases and the like because the use of such a monoclonal antibody can directly suppress the activity of human .alpha..sub.2 -PI to inhibit fibrinolysis of plasmin and promote fibrinolysis by plasmin.
It is known that in clinical medicine, the level of human .alpha..sub.2 -PI in the blood decreases in disorders of parenchymatous liver cells, and it has been reported that the level of human .alpha..sub.2 -PI in blood shows a marked decrease in decompensated liver cirrhosis and fulminant hepatitis [see N. Aoki & T. Yamanaka: Clinica Chimica Acta, 84, 99-105 (1978)].
Recently, the chemical, physical and biological properties of human .alpha..sub.2 -PI have been elucidated in detail, and it has been found that human .alpha..sub.2 -PI specifically controls and regulates the fibrinolytic mechanism of plasmin and performs an important action on this mechanism [see, for example, N. Aoki & P. C. Harpel: "Seminars in Thrombosis and Hemostasis, 10, 24-41 (1984)].
Accordingly, the provision of a monoclonal antibody capable of blocking the reactive site of human .alpha..sub.2 -PI specifically would enable the amount of human .alpha..sub.2 -PI in blood to be accurately and easily measured, and would be quite useful for the prevention and diagnosis of various diseases.
The only literature reference which discloses a monoclonal antibody to human .alpha..sub.2 -PI is Belgian Patent Specification No. 896,543 laid-open on Aug. 16, 1983. This patent specification states that 23 monoclonal antibodies to .alpha..sub.2 -PI which can be classified into 11 antibody groups having different epitope specificities were obtained, but fails to determine which epitopes of .alpha..sub.2 -PI these monoclonal antibodies will specifically recognize and combine with.
It is a primary object of this invention therefore to provide a highly specific monoclonal antibody or its fragment having the function of specifically blocking the reactive site of human .alpha..sub.2 -PI which inhibits the fibrinolytic activity of plasmin and thereby suppressing the action of human .alpha..sub.2 -PI to inhibit the fibrinolytic activity of plasmin.
Another object of this invention is to provide a hybridoma which secretes such a monoclonal antibody and a process for its production.
Still another object of this invention is to provide a method of immunologically assaying .alpha..sub.2 -PI in an assay sample by using the aforesaid monoclonal antibody, and a reagent which can be used in this method.
Yet another object of this invention is to provide a selective adsorbent for .alpha..sub.2 -PI using the aforesaid monoclonal antibody, and a method of separating or recovering .alpha..sub.2 -PI by using the adsorbent.